# odbc {#table_functions-odbc}

Returns table that is connected via [ODBC](https://en.wikipedia.org/wiki/Open_Database_Connectivity).

```
odbc(connection_settings, external_database, external_table)
```

Parameters:

- `connection_settings` — Name of the section with connection settings in the `odbc.ini` file.
- `external_database` — Name of a database in an external DBMS.
- `external_table` — Name of a table in the `external_database`.

To safely implement ODBC connections, ClickHouse uses a separate program `clickhouse-odbc-bridge`. If the ODBC driver is loaded directly from `clickhouse-server`, driver problems can crash the ClickHouse server. ClickHouse automatically starts `clickhouse-odbc-bridge` when it is required. The ODBC bridge program is installed from the same package as the `clickhouse-server`.

The fields with the `NULL` values from the external table are converted into the default values for the base data type. For example, if a remote MySQL table field has the `INT NULL` type it is converted to 0 (the default value for ClickHouse `Int32` data type).

## Usage example

**Getting data from the local MySQL installation via ODBC**

This example is checked for Ubuntu Linux 18.04 and MySQL server 5.7.

Ensure that unixODBC and MySQL Connector are installed.

By default (if installed from packages), ClickHouse starts as user `clickhouse`. Thus you need to create and configure this user in the MySQL server.

```
sudo mysql
mysql> CREATE USER 'clickhouse'@'localhost' IDENTIFIED BY 'clickhouse';
mysql> GRANT ALL PRIVILEGES ON *.* TO 'clickhouse'@'clickhouse' WITH GRANT OPTION;
```

Then configure the connection in `/etc/odbc.ini`.

```
$ cat /etc/odbc.ini
[mysqlconn]
DRIVER = /usr/local/lib/libmyodbc5w.so
SERVER = 127.0.0.1
PORT = 3306
DATABASE = test
USERNAME = clickhouse
PASSWORD = clickhouse
```

You can check the connection using the `isql` utility from the unixODBC installation.

```
isql -v mysqlconn
+---------------------------------------+
| Connected!                            |
|                                       |
...
```

Table in MySQL:

```
mysql> CREATE TABLE `test`.`test` (
    ->   `int_id` INT NOT NULL AUTO_INCREMENT,
    ->   `int_nullable` INT NULL DEFAULT NULL,
    ->   `float` FLOAT NOT NULL,
    ->   `float_nullable` FLOAT NULL DEFAULT NULL,
    ->   PRIMARY KEY (`int_id`));
Query OK, 0 rows affected (0,09 sec)

mysql> insert into test (`int_id`, `float`) VALUES (1,2);
Query OK, 1 row affected (0,00 sec)

mysql> select * from test;
+--------+--------------+-------+----------------+
| int_id | int_nullable | float | float_nullable |
+--------+--------------+-------+----------------+
|      1 |         NULL |     2 |           NULL |
+--------+--------------+-------+----------------+
1 row in set (0,00 sec)
```

Retrieving data from the MySQL table in ClickHouse:

```sql
SELECT * FROM odbc('DSN=mysqlconn', 'test', 'test')
```
```text
┌─int_id─┬─int_nullable─┬─float─┬─float_nullable─┐
│      1 │            0 │     2 │              0 │
└────────┴──────────────┴───────┴────────────────┘
```

## See Also

- [ODBC external dictionaries](../../query_language/dicts/external_dicts_dict_sources.md#dicts-external_dicts_dict_sources-odbc)
- [ODBC table engine](../../operations/table_engines/odbc.md).

[Original article](https://clickhouse.yandex/docs/en/query_language/table_functions/jdbc/) <!--hide-->